1. Field of the Invention
This invention relates to a charged-particle beam optical apparatus for the reduction imaging of a mask on a specimen to be examined which comprises a beam source for illuminating the mask, a condenser lens system comprising a plurality of condenser lenses for generating a ray bundle which strikes the mask as a probe, a beam deflection system located ahead of the last of the condenser lenses in the direction of the beam path, and a projection lens system including a long focal length intermediate lens and a short focal length imaging lens which are spaced apart by a distance which is equal to the sum of their focal lengths, the mask being located in the front focal plane of the intermediate lens.
2. Description of the Prior Art
An apparatus of the foregoing type is described in the paper by Heritage, J. Vac. Sci. Technol., Vol. 12 (1975), pages 1135 et seq. It is used particularly for generating patterns on semiconductor wafers in the fabrication of integrated circuits. The imaging scale in such an apparatus is about 10:1 and the image of the mask is projected on the specimen so that (a) the entire mask area to be imaged is simultaneously illuminated and (b) so that all points on the mask are simultaneously imaged by the projection lens system of the apparatus. In order to adjust the mask and the specimen relative to each other, the excitation of the condenser lens system must be increased so that the beam source (assuming the apparatus lenses are ideal) is imaged by the projection lens system as a point in the mask plane and as a point in the specimen plane. A deflection system is located above the mask and is excited so that the point-shaped probe formed by the beam scans a test opening in the mask plane. This scanning generates an image of the test opening in the specimen plane, and a detector located near the specimen registers secondary electrons released at the specimen (which may be a semiconductor wafer). The signal generated by the detector is transmitted to a monitor on the picture screen of which an image showing an adjustment marking on the specimen and an image of the test opening are displayed. To effect adjustment, the mask or the specimen is shifted until the images of the test opening and the adjustment marking coincide. After adjustment, the excitation of the condenser lens system must be reset so that the mask is integrally illuminated for imaging its entire area on the specimen. Exact resetting of the excitation of the condenser lenses is, however, very difficult if the condenser lens adjusted includes an iron shell, which is usually the case.
Since the condenser lens system of the apparatus does not consist of ideal lenses, but instead causes errors, the image of the beam source in the mask plane is not a point. As a result, the test opening in the mask is imaged in the specimen plane with an unavoidable lack of definition. The accuracy of the adjustment is accordingly limited.